1. Field of the Invention
The present invention concerns a powder mixture used for powder metallurgy comprising a metal powder such as an iron powder or a steel powder as a substrate and a powder of alloying element, graphite, phosphorus, etc. mixed therewith as the ingredient for improving the physical property, in which a copolymer ingredient is incorporated at a specific composition ratio as a binder for suppressing the segregation of the physical property improving ingredient as described above without deteriorating the physical property of the metal powder as the substrate and suppressing dusting upon handling the powder.
2. Description of the Prior Art
In powder metallurgy using a metal powder such as an iron powder or a steel powder as the main starting material, a powder of alloying element such as copper, nickel, chromium or molybdenum, inorganic ingredient such as graphite, phosphorus or manganese sulfide (MnS) (hereinafter sometime referred to collectively as the physical property improving ingredient) for is often blended improving the physical property such as strength or fabricability of the sintering product. In this case, grain size, specific gravity, etc. are usually different considerably between the metal powder as the substrate and the powder of the physical property improving ingredient. For instance, if the metal powder as the substrate is an iron powder or a steel powder and the powder for the physical property improving ingredient is graphite or phosphorus, since the difference in the specific gravity is extremely great this tends to cause segregation in the course of handling from the mixing to molding steps causing to worsen the property and the homogenity of the sintering product.
As the means for preventing such segregation, there has been proposed a method of depositing a graphite powder, etc. to an iron or steel powder, etc. using an organic binder as disclosed in, for example, Japanese Patent Laid-Open Sho 56-136901 and Sho 63-103001.
However, the organic binder disclosed in Japanese Patent Laid-Open Sho 56-136901 is hydrophilic and, accordingly, involves a problem that it absorbs moisture to worsen the flow rate during storage or accelerates rusting of the metal powder as the substrate thereby worsening the quality of the sintering product. The organic binder as disclosed in Japanese Patent Laid-Open Sho 63-103001 shows a stronger effect of increasing the bonding force between iron or steel powder with each other rather than the bonding force of the iron or steel powder, for example, to the graphite powder and, accordingly, it has an insufficient effect for preventing the segregation of graphite. Then, a great amount of the binder has to be blended in order to attain an excellent effect and bonding (blocking) is inevitable between iron or steel powder with each other, making it necessary for a repulverization or sieving step after mixing and drying.